As television entered its Golden Age in the early 1950s, Gelbart segued to the new medium, first as a staff writer on "The All-Star Revue" (NBC, 1950-53) and then alongside Neil Simon, Mel Tolkin, Woody Allen and Mel Brooks on "Caesar's Hour" (NBC, 1955-57). Throughout the 50s, he continued to provide amusing material for Red Buttons, Patrice Munsel and Pat Boone and garnered awards for his work on the NBC variety special "The Art Carney Show" (1959). At the dawn of the 60s, Gelbart branched out to write the libretto for the ill-fated musical "The Conquering Hero" (1960) but hit pay dirt with his second effort. With Burt Shevelove, Gelbart modernized the comedies of Plautus and created the book for one of Broadway's best musicals, "A Funny Thing Happened on the Way to the Forum". The original 1962 production, with a score by Stephen Sondheim, produced by Harold Prince and directed by George Abbott, won six Tony Awards, including one for its book writers and one for star Zero Mostel. (It has been successfully revived in 1972 with Phil Silvers and 1996 with Nathan Lane.)

Around the time of his stage success, Gelbart co-wrote the screenplays for his first features. "The Notorious Landlady" (1962), scripted with Blake Edwards, was an enjoyable thriller that found Jack Lemmon investigating the mysterious death of Kim Novak's husband. With Carl Reiner, he penned "The Thrill of It All" (1963), a spoof of TV commercials with James Garner and Doris Day. That same year, Gelbart and his family decamped for London where he spent nine years, during which he worked on several film scripts, the best of which was "The Wrong Box" (1966), co-written with Burt Shevelove. Returning to the USA in the early 70s, the writer worked on what was to be a critical--and very lucrative--project, turning Robert Altman's 1970 black comedy "M*A*S*H*" into a weekly series. Working with Gene Reynolds, Gelbart spent four years (1972-76) shepherding the series, a mixture of comedy and drama that won several awards and eventually ran 11 seasons.

When he left "M*A*S*H*", Gelbart returned to Broadway with "Sly Fox" (1976), an adaptation of Ben Johnson's "Volpone" that starred George C Scott. He also resumed his film career nabbing an Oscar nomination for his script to the genial "Oh, God!" (1977), in which supermarket manager John Denver is visited by the Almighty (personified by George Burns). Gelbart co-wrote the pastiche of a double bill "Movie Movie" (1978), Stanley Donen's paean to 1930s Hollywood that featured a Technicolor musical and a black and white boxing drama. Disappointed with the final version of "Rough Cut" (1980), a Burt Reynolds caper, he eschewed credit in favor of the pseudonym Francis Burns. The troubled production of "Tootsie" (1982), for which he provided the original story and script, yielded a second Academy Award nomination. Gelbart frequently clashed with director Sydney Pollock and was unhappy to share final credit with Murray Schisgal (although others were also said to have tweaked the script). His last feature to date was the dismal reteaming with Stanley Donen, "Blame It on Rio" (1984).

In 1980, Gelbart attempted the intriguing "United States" (NBC), a serio-comic look at contemporary marriage starring Beau Bridges and Helen Shaver. Audiences were not primed for such an atypical series, one that abandoned a laugh track, dealt with mundane issues and failed to wrap things up in a happy ending. Ahead of its time, the series had its champions, but failed to earn high ratings and was canceled after two months. Similarly, the sequel "AfterMASH" (CBS, 1983-84), which followed the post-war exploits of three supporting players from "M*A*S*H*", also failed to win viewers. Returning to the stage, Gelbart had two very different shows on the boards during the 1989-90 theatrical season. The short-lived "Mastergate" was a satirical examination of political scandals (i.e., Watergate, the Iran-Contra hearings) that closed quickly, although a 1992 television version for Showtime fared better. Gelbart had a bona fide hit, though, with the musical "City of Angels". With a score by Cy Coleman and David Zippel, "City of Angels" was both a pastiche of films noir and a behind-the-scenes skewering of Hollywood. It earned Gelbart his second Tony Award as well as five others including the Best Musical trophy.

The 90s saw Gelbart back on the small screen, this time in tandem with cable channels. He penned the highly entertaining adaptation of "Barbarians at the Gate" (HB0, 1993), about the rise and fall of Nabisco chairman F Ross Johnson. Gelbart then turned his sights on tabloid media moguls in the black comedy "Weapons of Mass Distraction" (HBO, 1997), He also served as executive producer of "Fast Track", a 1997 Showtime series set in the world of professional stock car racing. As the 90s were closing, the busy writer was juggling a number of projects, including writing drafts of the script for the proposed film version of "Chicago" and writing an HBO series about a Beverly Hills plastic surgeon. Gelbart also contributed to the screenplay for the pallid remake of "Bedazzled" (2000).